The present invention relates to an image pickup apparatus. Particularly, this invention relates to an image pickup apparatus, such as, a video camera equipped with a charge-coupled device (CCD).
A charge-coupled device used for video cameras has a light-receiving section 100 and a light-blocking section 101, arranged as shown in FIG. 1A.
The light-receiving section 100 receives light from an object via lens to generate an analog video signal which is converted into a digital video signal by analog-to-digital conversion.
On the other hand, the light-blocking section 101 blocks the light by an aluminum plate, for example. The light-blocking section 101 is also provided with a horizontal black-reference detector to detect horizontal optical-black (OB) data.
The OB data is a black-reference (optical black) signal for indicating reference of optical black used for displaying pictures. The black-reference signal is used to clamp the reference level of the digital video signal for achieving a high reference-level stability.
The OB data is supplied to a digital processor installed in a video camera. The digital processor accumulates the OB data in the horizontal direction and calculates the average value of the accumulated OB data for each predetermined number of pixels in the horizontal direction. The digital processor further accumulates the average values in the vertical direction and calculates the average value of the accumulated average values for each predetermined number of pixels in the vertical direction. The average value obtained in the vertical direction is output as the final OB data.
The number of scanning lines for accumulation in the vertical direction is 128 lines for television signals according to the NTSC (National Television System Committee) system.
In detail, as illustrated in FIG. 1B, the OB data are accumulated for each group of Nh pixels (Nh=16, for example) after N pixels from the rising edge of a horizontal synchronizing signal (HD) for calculation of the average value of the OB data in the horizontal direction.
The calculation of average value in the horizontal direction starts at the Y-th line after the rising edge of a vertical synchronizing signal (VD), and the calculation of average value in the vertical direction starts at the Nv-th line (Nv=128, for example) after the Y-th line.
Performance specifications of a driver to drive the CCD decides at which clock after the rising edge of the HD the OB data calculation starts in the horizontal direction and after how many HDs from the VD the OB data calculation starts in the vertical direction.
The number of times for accumulation in the horizontal direction is preferably smaller than the number of pixels located in the light-blocking section 101 and the accumulation is preferably applied to pixels the number of which is 2n (n: a positive integer). Moreover, the accumulation in the horizontal direction preferably starts at the m-th line (m: a positive integer) so that the accumulation can be applied to the center pixels from which the OB data can be detected on the light-blocking section 101.
The accumulation in the vertical direction is, for example, applied to 128 lines as discussed. The number “128” is the maximum number of lines because the value that comes next to 128 (=27) is 256 (=28) that is larger than the number of scanning lines for TV signals according to the NTSC system. Moreover, the accumulation in the vertical direction preferably starts after several lines from the rising edge of the VD so that the accumulation can be applied to the center pixels from which the OB data can be detected on the light-blocking section 101.
The values Nh and Nv discussed above are preferably 2n because division in digital processing can be performed by rounding-down of 2n. The average value of OB data on pixels in the light-blocking section 101 (16≦pixels≦32, for example) is obtained by accumulation on 16 pixels and rounding-down of the lower 4 bits of the accumulated data. The average value for pixels equal to 32 or more can be obtained by accumulation for the 32 pixels and 5-bit shifting of the accumulated data.
The digital video signal and the OB data obtained as above are supplied to a digital processor for obtaining the difference between the digital signal and data and performing digital clamping processing to cancel the difference. In other words, the reference (black) level of the digital video signal is adjusted on the basis of the OB data.
One type of video cameras that has recently been on the market is capable of high-speed photographing in which signals are read from the CCD several times for one field period (VD) for photographing several pictures within one field period.
Such a high-speed photographing mode further accumulates the horizontal OB data obtained as discussed above in the vertical direction to calculate average OB data for further accurate black level adjustments for pictures.
However, the digital clamping processing discussed above cannot be applied to such a high-speed photographing-type of video cameras for several reasons.
Digital clamping processing for video data of “n” pictures within one field period (VD), for which signals are read from CCD several times for one VD from the present vertical synchronizing signal to the succeeding synchronizing signal, would generate noises on the period of OB data to one video data per one VD, which overlaps the period of “n” video data per one VD.
Moreover, the larger the number “n”, the shorter the period of digital clamping processing for video data of “n” pictures per one VD. High-speed CPUs are thus required for such high-speed digital clamping processing.
High-speed CPUs, however, consume a lot of power to generate heat; and hence require cooling mechanism, thus causing difficulty in compactness of camera size, shortage of battery life-time, and cost-up in fabrication.